1. Field of the Invention
The present invention relates to semi-conductor wafer handling equipment, and, more particularly, to a method of and apparatus for selectively removing semi-conductor wafers from quartz processing boats.
2. History of the Prior Art
In the manufacture of semi-conductor devices, such as integrated circuits, a plurality of devices are formed on a single wafer of silicon material. Each wafer is typically circular and on the order of 6 inches in diameter. The wafers are put through a number of sequential processing steps, including coating them with photo-resists, exposing them to the optical patterns formed on photo masks, exposing them to both liquid and gaseous treating environments, and passing them through high temperature baking operations.
The processing of a silicon wafer containing a plurality of semi-conductor devices requires a high degree of cleanliness and sterility in the environment in order to produce acceptable devices. The ability of a semi-conductor device to perform satisfactorily from both an electrical and a mechanical standpoint depends upon the nature and quality of the materials forming the various layers of the device. The chemical composition of these materials must be extremely pure. The introduction of any foreign matter into the environment where the wafers are being processed results in a decrease in the "yield" of the wafer. The yield is the number of devices that can pass the required electrical tests of the device after the processing has been completed. This is usually expressed as a fraction of the total number of devices processed on the wafer that did pass the required tests. Thus, the higher the purity of the processing environment and the processing and wafer handling techniques used in manufacturing the semi-conductor devices, the greater the yield and hence the greater the finanical return to the manufacturer.
The semi-conductor wafers are generally handled between processing steps in inert plastic frames containing a plurality of vertically arranged dividers. Each divider defines a pocket to receive a silicon wafer and holds it securely in a vertical orientation while isolating it from adjacent wafers. The plastic frame and its dividers are together referred to as a wafer "cassette." A typically industrial cassette may hold on the order of 25 wafers for processing. The cassettes are physically moved by operators from one processing station to another wherein the cassettes are placed in indexing mechanisms that generally form a part of the processing machinery of each station. Each wafer is then automatically removed from the cassette for processing and returned by the indexing mechanism after the processing step of each station has been completed.
It is sometimes necessary during the processing operation to occasionally remove selected wafers from the cassette in order to perform quality assurance tests, to read a serial number on a wafer, or for other production or quality control reasons. In conventional wafer cassettes holding on the order of 25 wafers, there is generally sufficient spacing between adjacent wafers for an operator to grip a single wafer along its edge with a pair of tweezers or a "vacuum wand." In this manner, the wafer may be gently removed from the cassette for inspection or testing. Removing an individual wafer from a cassette is a very delicate operation because a wafer can only be touched along a small area within approximately 1/8" from its edge. This is necessary in order to avoid physically damaging the semi-conductor devices, referred to as "dies," being formed on the wafer. In addition, if the surface of a wafer touches any of the adjacent wafers during the removal operation, scratches can occur, resulting in destruction of one or more of the dies being formed on the wafer and, thus, reducing the yield of the wafer.
During certain manufacturing operations, the semi-conductor wafers being processed are kept very close together, i.e., closer to one another than in the above described conventional wafer cassettes. For example, in a baking operation in which the temperature of the oven may be raised to the order of 1800 degress Celsius, the wafers are kept very close together for even heat distribution across the entire group of the wafers being processed. It is generally required that, because of the high degree of control necessary in certain processes, there be temperature variations of no more than 5 degrees Celsius between any one of the adjacent wafers being processed. The more closely the wafers are packed together, the higher the probability that they will all be raised to the same temperature, resulting in a high degree of homogeneity in the baking processing step.
In order to carry semi-conductor wafers through a high temperature baking operation, such as that described above, the wafers must be placed in a specially designed carrier that is generally referred to as a quartz "boat." The quartz boat is designed to withstand high temperatures and also provide a high degree of stability in supporting the wafers so that there is relatively little chance that any one of the tightly packed wafers will touch an adjacent wafer. Boats formed of precisely machined quartz glass material are generally employed for such processing steps. However, when semi-conductor wafers are packed extremely close together in such a processing boat, spacing therebetween can be reduced to the order of 1/32". It thus becomes virtually impossible for even the most skilled operator to selectively remove any one wafer from the group by conventional techniques in order to inspect and/or test it without scratching the surface of an adjacent wafer. Selective removal of the wafers for testing, such as batch baking, is very important to insure that the processing of the dies on the multiple wafers was successful. In the event some error in the processing operation through which the wafer has just passed is not revealed until after the wafers have been subsequently sent through additional processing steps, substantial expense will be incurred to treat the defective wafers. In addition, it is important to know if a processing step is not working properly in order to avoid processing additional wafers through that step without making the necessary adjustments and/or taking corrective action.
Prior art techniques have sought to provide automatic equipment for engaging select wafers from tightly packed groups in a carrier. These techniques generally include gripping a wafer from the top and then very accurately removing it from between adjacent wafers. Such systems have inherent disadvantages. Anytime a wafer is gripped in order to lift it from a carrier, both substantial contact and pressure are applied to the wafer as it is pulled upwardly. Any increase in the area of contact with the wafer or pressure on the wafer has been shown to result in a release of contaminating particles and related impurities from the equipment. These impurities would move downwardly onto the surface of the wafer and contaminate one or more of them. It has also been shown that when metal or plastic comes in contact with the wafer the friction between the surfaces alone causes contamination.
Other prior art systems have sought to lift select wafers from below the cassette and raise the wafer upwardly from within their neighboring wafers for facilitating the requisite access thereto. Such systems have not, to date, proven to be sufficiently accurate to provide a practical solution to the problems enumerated above. In addition, the high degree of accuracy necessary for removing wafers from the tightly packed boats requires a high degree of precision in the positioning of the boat with respect to the handling equipment. This aspect alone has limited the accuracy of certain prior art systems.
It would thus be desirable to provide a system for selectively removing semi-conductor wafers from a tightly packed processing boat without the need to grip the wafers from above or apply any undue pressure to them. In addition, it would be desirable to have the system programmable to selectively remove a particular wafer from a group of wafers regardless of the spacing between them, the centers upon which they are located, or the size and configuration of the processing boat within which they are being handled. It would also be an advantage to be able to quickly and accurately remove each wafer for testing purposes with a minimum of contamination of the wafers.